- Written By: Shogo MutoDirected By: Satoshi Morota
Sento: With the immense power of the Pandora Box in his possession, the extraterrestrial entity Evolt has begun to take over the country. The memories of Katsuragi Takumi resurface in Kamen Rider Build AKA Kiryuu Sento, and he clashes with Evolt's agent, Kamen Rider Mad Rogue.
Ryuga: Sento's finally back!
Sento: It's you! The simpleton musclebrain who loves protein way more than a balanced diet, Banjou Ryuuga!
Ryuga: Is that seriously how you remember me?!
Sento: And now enjoy episode 40, which will leave even Banjou saying "Holy crap" and "He's unstoppable."
Our genius physicist is back in action! However, Evolt seems to be undeterred. What could this mean for our protagonists?
- Brutal Honesty: Only Misora is willing to speak up and say that Gentoku's choice in wardrobe is awful.
- The Bus Came Back: After being absent for almost 20 episodes due to being captured by Seito forces, Yoshiko Tajimi, the imprisoned Prime Minister of Hokuto, finally returned and became Evolt's pawn against Build, in the form of Owl Lost Smash..
- Chekhov's Gunman: The wife of the human trafficker from #3 is brought up again when Sento finds a picture of her among Shinobu's belongings. This is important because she reveals that Shinobu Katsuragi is still alive and that she ferried him to some place the previous day.
- Curbstomp Battle: Sento delivers one to Utsumi in Genius form, trivially crushing him without any effort in a few moves.
- Everyone Has Standards: When Takumi speaks up and says Sento won't find anything of value among Shinobu's possessions, Takumi also questions why his father would even create the Lost Bottles considering how dangerous they are and what they can do to a person.
- Fusion Dance: A Lost Smash is created when someone with a Lost Bottle uses it and combines with nearby Clone Smash.
- Good Costume Switch: Gentoku decides to change his wardrobe now that he's part of nascita, but his choice in clothing (custom made no less) leaves the protagonists somewhere between afraid and shocked.
- Heroic Second Wind: When Sento starts his rhetoric on how the Kamen Riders fight for love and peace, Kazumi manages to pull himself together to keep fighting late in the episode after being downed by Utsumi's Evoltech Attack and effectively bleeding out after Utsumi forced Kazumi to be treated with even more Nebula Gas like the Washio Bros. Kazumi's spur of the moment usage of the Dragon Sclashjelly allowed him power up properly and give him another Twin Breaker to fight with, and the resulting level up is enough to fight Utsumi as Mad Rogue on equal footing.
- It Amused Me: Evolt decides to create Lost Smash, which are the supposed ultimate form of Smash possible, using the Lost Bottles he took back from Kazumi and whatever humans he feels like. When questioned on why by Utsumi, Evolt just says it is for fun, and possible karmic justice for self-absorbed humans to destroy one another before the planet itself is destroyed. He also decides to leave the other Kamen Riders alive because it would be a Dramatic Irony if they lost in battle and ended up as Lost Smash.
- It May Help You on Your Quest: When Ryuga couldn't convince Sento or Kazumi from going to Hokuto to look for clues, Ryuga gives Kazumi a Dragon Sclashjelly before going back to nascita. Whether it is a Plot Hole or not remains to be seen.
- My God, What Have I Done?: When Prime Minister Tajimi recovers after being defeated as the Fukuro Lost Smash, she has a minor freak out and asks for forgiveness for what she has done.
- Plot Hole: An In-Universe example, as the protagonists discover a missing link when Sento finds a picture with Reika Kine (aka the first Flying Smash in series and human trafficker) that has a Guardian in the background since Guardians were not publicly in service until 5 years after the Sky Wall Disaster, and thinks about how this has any relation to his father Shinobu since the picture itself had to have been taken some time after his now alleged death that was supposed to have happened six months after the Sky Wall Disaster.
- The Reveal: Takumi reveals that the Lost Bottles were created by Shinobu, his father, and if Sento couldn't find any files about them, then Evolt probably deleted those files.
- Later on, Takumi reveals that the Lost Bottles make their users more violent and easier to manipulate, but doesn't understand why Shinobu would make them.
- Talking to Themself: Sento and Takumi are now able to talk to each other, but only Sento can hear him.